LONDON (AP) ― Thirty years ago, artist Jean-Michel Basquiat secretly signed one of his paintings in invisible ink, says Sotheby’s auction house, which discovered the hidden autograph as it was preparing the painting for sale.

Sotheby’s experts uncovered the secret this month as they were examining “Orange Sports Figure,’’ which goes on sale Wednesday. The vibrant image of an abstract crowned figure is estimated to be worth between 3 million pounds and 4 million pounds ($4.7 million and $6.3 million).

Basquiat, a graffiti artist who became a 1980s art star, signed relatively few of his canvasses. But Sotheby’s said ultraviolet light revealed the artist’s name and the date 1982 beneath the work’s layers of acrylic and spray paint.

“The signature just popped out,’’ Cheyenne Westphal, head of contemporary art at Sotheby’s Europe, said Tuesday.

She said staff were initially “surprised, astonished and puzzled’’ by the signature, which appears to have been written in the type of pen used to mark banknotes.

“Nobody else probably ever knew about this invisible inscription, and the prospect that he might have left other invisible writings on his canvasses that are only visible under ultraviolet light is very exciting,’’ she said.

Westphal said she knew of no other invisible signature on a Basquiat work.

Basquiat’s paintings are often covered in words and doodles. He signed some paintings with a crown, others with his graffiti alter ego SAMO ― but relatively few with his full name.